[x86] add tests for convert-FP-to-integer with constants; NFC

We don't constant fold any of these, but we could...but if we
do, we must produce the right answer.

Unlike the IR fptosi instruction or its DAG node counterpart 
ISD::FP_TO_SINT, these are not undef for an out-of-range input.

llvm-svn: 329100